Ukrainian president Petro Poroshenko has promised to work on an urgent ceasefire plan to defuse his country's separatist conflict following late-night talks with Russian leader Vladimir Putin.

The first negotiations between the two leaders since June were described by Mr Putin as positive, but he said it was not for Russia to get into the details of truce terms between the Kiev government and rebels.

"We can only contribute to create a situation of trust for a possible, and in my view, extremely necessary, negotiation process."

Mr Poroshenko said two hours of one-to-one talks were "very tough and complex".

"A roadmap will be prepared in order to achieve as soon as possible a ceasefire regime which absolutely must be bilateral in character," he said.

Despite the positive tone, it remained unclear how the rebels would respond to the idea of a ceasefire, how soon it could be agreed, and how long it might stick.

And with Mr Putin insisting the details were an internal matter for Kiev, there was no sign of progress on a fundamental point of disagreement: Ukraine's charges that Moscow is sending arms and fighters to help the rebels, and Russia's adamant denials.

The Minsk talks, preceded by six hours of wider negotiations with top European Union officials and the presidents of Belarus and Kazakhstan, were aimed at ending five months of conflict that has killed at least 2,200 people.

"We all wanted a breakthrough," Belarusian president Alexander Lukashenko told reporters after the multi-lateral part of the talks had finished.

"But the very fact of holding the meeting today is already a success, undoubtedly.

"The talks were difficult. The sides' positions differ, sometimes fundamentally ... Everybody agreed on the need to de-escalate and free hostages."

Kiev claims Russians entered Ukraine on 'secret mission'

Mr Poroshenko and Mr Putin shook hands in Minsk just hours after Kiev said a group of Russian servicemen had intentionally crossed into Ukrainian territory on a "special mission", contradicting Russian assertions they got there by accident.

Russian military officials said 10 Russian paratroopers, who have been captured by Ukraine's army in the east of the country, crossed the border by mistake.

Russia's defence ministry said the soldiers were patrolling the border and crossed into Ukraine accidentally at an unmarked section.

But Ukraine's military spokesman, Andriy Lysenko, dismissed that, mocking the idea that "the paratroopers got lost like Little Red Riding Hood in the forest".

"This wasn't a mistake, but a special mission they were carrying out," Mr Lysenko said in a televised briefing.

Moscow has consistently denied claims it has been sending weapons and fighters to help the separatists in Ukraine.